Under certain conditions, when the air is heavily charged with dust and contaminants, it is extremely important to ensure absolute protection for the engine, regardless of what happens to the main filter itself.
Until now, the large filter housings have been composed of a single stage of a plurality of filter elements installed in parallel banks or modules, but these elements are installed in such a way that if one of them fails to perform to requirements, the entire protected system loses its protection.
A second stage is sometimes installed downstream from the first stage, as a back-up in case of failure of the first stage. The second stage generally consists of a costly rack, in which another plurality of cleaning elements is installed in the central plenum upstream from the filter outlet. Such a system presents various drawbacks. The second stage must incorporate a large surface area in order to minimize its pressure loss and to insure cleaning efficiency in the event of failure of the first stage, as the air velocity is generally high due to the reduced available space. A large filtration area provides a large dust capacity which increases the need for high efficiency in order to minimize, in the event of an accident, the quantity of dust ingested by the engine.
High efficiency means that during normal performance all the dust and contaminants passing the first stage are trapped by the second stage. In this situation a high retention capacity upstream from the second stage is emphasized.
In the prior art devices if one element leaks, the whole safety stage risks being contaminated and clogged. In order to detect such clogging, the differential pressure through the second stage must be monitored to signal an alarm whenever necessary. An access to the second stage is required in order to allow servicing of the cleaner device. When maintenance is conducted from the dirty air side, the cost of the second stage is very high. If the servicing of the cleaner device is carried out from the clean air side, the cost is lower but the danger of contamination of the engine is increased and loose tools, nuts or removed dust may reach the engine. Moreover, it is difficult to locate the leaking element because such often is evident only by dirt traces on metal surfaces when the main filter has a small fissure through which part of the retained dust and contaminants escape.
Generally a leak is detectable only when the pressure loss of the entire system is excessive. The turbine or engine, protected by the cleaner device, must be stopped and both the primary stage and the safety stage have to be replaced. Due to the high filtering efficiency of the second stage, its differential pressure is constantly and slowly increasing because all of the contaminants passing the first stage are picked up by it. It is difficult to observe a slow increase of differential pressure in a short time, especially when no recording is made. The leakage continues until the first stage or faulty element is replaced at the time of normal servicing of the entire cleaning system. The existing safety cleaning systems are very expensive and complicated since, as explained above, the use of a second stage filter for safety significantly increases the cost, as well as, the bulkiness of the system and more particularly, the servicing costs and the risks of operating problems are increased.